It’s one of life’s special moments: a child finds a fat caterpillar, puts it in a jar with a twig and a few leaves, and awakens one day to find the caterpillar has disappeared and an elegant but apparently lifeless case now hangs from the twig.

The prevailing model for planetary accretion, also called fractal assembly, and dating back as far as the 18th century, assumes that the Solar System’s planets grew as small grains colliding chaotically, coalescing into bigger ones, colliding yet more until they formed planetesimals.

Here’s a riddle: What’s the difference between a tick and a lion? The answer used to be that a tick is a parasite and the lion is a predator. But now those definitions don’t seem as secure as they once did.

In 2011 — to the consternation of women everywhere — a systematic review of randomized clinical trials showed that routine mammography was of little value to younger women at average or low risk of breast cancer.

A pivoted catch designed to fall into a notch on a ratchet wheel so as to allow movement in only one direction (e.g. on a windlass or in a clock mechanism), or alternatively to move the wheel in one direction.